Don't print escaped strings for the default value of string flags.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

flag.go

diff --git a/flag.go b/flag.go
index 4258f45..d9aee57 100644
--- a/flag.go
+++ b/flag.go
@@ -514,7 +514,7 @@
 		if len(flag.NoOptDefVal) > 0 {
 			switch flag.Value.Type() {
 			case "string":
-				line += fmt.Sprintf("[=%q]", flag.NoOptDefVal)
+				line += fmt.Sprintf("[=\"%s\"]", flag.NoOptDefVal)
 			case "bool":
 				if flag.NoOptDefVal != "true" {
 					line += fmt.Sprintf("[=%s]", flag.NoOptDefVal)